The present invention resides in a process for the production of a carbonate of aluminum and an alkali or alkali earth metal, in particular a basic carbonate of aluminum and sodium of the dawsonite type. The following description of the invention and the carrying out of the process is therefore limited to dawsonite.
The composition of the dawsonite (in the following the basic carbonate of aluminum and sodium is expressed as dawsonite) can be represented by the following equivalent formulae: EQU Al.sub.2 O.sub.3.Na.sub.2 O.2CO.sub.2.2H.sub.2 O or AlNaO(OH)HCO.sub.3 or as AlNa.sub.3 (CO.sub.3).sub.3.2Al(OH).sub.3
All three formulae are found frequently in technical literature.
A number of applications has been found for dawsonite e.g. medicaments to counter hyperacidity of the stomach, filler material for light-sensitive papers, additive for lacquers and in the paint manufacturing industry. Recently dawsonite was proposed as a filler material with reinforcing and/or flame retarding properties for polymeric synthetic resins such as polystyrole, polypropylene or polyethylene. As far as its flame retarding properties are concerned, it was already known that a corresponding substance, in which the sodium was replaced by ammonia, would seem to be more suitable for this purpose.
Both natural and synthetic dawsonite occurs in the form of fine needles.
The known processes for producing aluminum-sodium carbonate can be divided into the following three groups:
(a) Reacting an aluminate solution with a sodium bi-carbonate solution (if desired also with a bi-carbonate solution of another alkali or alkali earth metal such as, for example, potassium or magnesium.
(b) Reacting an alumina gel with a bi-carbonate solution of an alkali or alkali earth metal.
(c) Reacting an aluminum salt with a suitable bi-carbonate or carbonate solution, in accordance with the desired molar ratio of Al/CO.sub.2.
Unfortunately the product obtained is impure to a greater or lesser extent. In particular, it contains various amounts of aluminum hydroxide.
It has been suggested, for example, that an initial aluminate solution with a high molar ratio of Na.sub.2 O/Al.sub.2 O.sub.3 e.g. 6 could be used as a means for improving the purity of the dawsonite. However, in this case the resultant mother liquors contain large amounts of sodium carbonate and bi-carbonate which are difficult to dipose of economically.
If, on the other hand, a "Bayer" aluminate solution is used as is usually the case, the dawsonite obtained is not only impure but also colored a strong yellow or brown. This discoloration can represent a significant disadvantage, depending on the application.
If an aluminum hydroxide is employed, it must be particularly active if it is to react more or less completely under the conditions used up to now. In other words the starting point must be an alumina gel, with all the well known disadvantages which are associated with this process.